Geothermal probes can be constructed coaxially or U-shaped. U-shaped geothermal probes comprise an inlet pipe which leads down into the ground and at a lower end is connected in a connecting region to an outlet pipe in a fluid-conducting manner. The heat transfer fluid, also called heat transfer liquid, thus flows down the inlet pipe, merges in the connecting region into the outlet pipe and flows up again in the same. In coaxial geothermal probes, the inlet pipe is a probe outer pipe and the outlet pipe is a probe inner pipe arranged within the probe outer pipe. Outside the probe inner pipe and within the probe outer pipe there is an annular space, also called heat transfer space, which forms a heat transfer region. The arrangement of the probe outer pipe with respect to the probe inner pipe in this case is coaxial. The connecting region with a coaxial geothermal probe is formed by an opening of the probe inner pipe so that the heat transfer fluid that is located in the probe outer pipe or in the annular space in this case can flow into the probe inner pipe.
On passing through the geothermal probe, a heat transfer between the heat transfer fluid and the ground takes place. The heat transfer substantially takes place by convection. Where the heat is emitted or absorbed depends on whether the geothermal probe is used for a cold process or a heat process. To this end, generic geothermal probes are arranged up to 100 meter deep in the ground, in individual cases even greater depths are realised.
The heat transfer fluid is conducted into the geothermal probe at an inlet.
When at the inlet heat transfer fluid is introduced into the geothermal probe the heat transfer fluid as a rule is forced by pressurisation to pass through the entire length of a geothermal probe twice, once in inlet direction down the inlet pipe and once against the inlet direction up the outlet pipe. The quantity of heat transfer fluid conducted through the geothermal probe per time is called volumetric flow rate. In the lower end of the geothermal probe the heat transfer fluid is again directed upwards through the probe inner part/outlet pipe and can be removed at a drain. The probe inner pipe can also be connected to the inlet and the probe outer pipe also to the drain.
The temperature difference between the heat transfer fluid flowing into and flowing out of the geothermal probe is called temperature gradient in the following. A heat flow or a heat output, heat in brief, is extracted from the ground as heat reservoir.
In the case of geothermal probes of less than 100 m length or depth, the temperature gradient between the introduced heat transfer fluid and the discharged heat transfer fluid generally amounts to a few degrees. Introduction values between −2° C. and 1° C. and discharge values between 2° C. and 5° C. are usual. The temperature gradient is relatively low and the temperature of the heat transfer fluid exiting from the geothermal probes does not yet correspond to a heat requirement as demanded for example for heating residential rooms. The heat output however can be rendered utilisable with the help of a heat pump, wherein the efficiency of a heat pump indicates how effectively a heat output supplied by the heat transfer fluid is converted into a heat requirement for heating. In a heat pump, a supplied heat output is utilised at a low temperature level in order to evaporate a heat medium which is located in a second fluid circuit, in an evaporator. In this case, the evaporator is a component in which the heat extracted from the geothermal probes is fed to the heat pump at a low temperature level. To this end, the heat transfer liquid discharged from the geothermal probes flows through the heat exchanger, passing its heat onto the second fluid circuit. Following this, the heat medium is supplied to a pump, which compresses the now gaseous heat medium, thus bringing it to a higher pressure level. In the process, the gaseous heat medium heats up and this heat that can be utilised for heating a residential room. On passing on its heat to the residential room, the heat medium cools down and condenses. In a choke, the pressure is again expanded to the lower pressure level. It is now again supplied to the evaporator of the heat pump and a heat pump circuit is thus provided. However, there are still other types of heat pumps which do not function as described above. These are well known to the person skilled in the art. The heat transfer fluid in the geothermal probes can be omitted when the heat medium itself circulates through the geothermal probes. The disclosure is not limited to geothermal heat circuits with two separate circuits.
Usually, multiple geothermal probes are employed in a geothermal probe heat circuit since the utilisable temperature difference of a geothermal probe is not usually adequate in order to evaporate the heat medium in the second fluid circuit. Although geothermal probes are well suited for absorbing heat from the ground in order to be incorporated in the process of a heat pump, there is nevertheless the desire to increase the temperature gradient that can be achieved by the discharge value of the heat transfer fluid that can be achieved with popular geothermal probes. Generally it can be said that a higher discharge value of the heat transfer fluid results in an improved efficiency since the heat pump then has to handle a lower temperature difference between discharge value, i.e. the temperature of the heat transfer fluid and the heat requirement.
The temperature of the ground in Germany for example from a depth of 15 meter is constantly approximately 10° C. throughout the year and it increases by approximately 120  C. for every further 30 meters. In particular in the case of geothermal probes with a length of under 70 m, the discharge value that can be achieved is frequently inadequate since the heat exchange does not take place effectively enough. It is additionally advantageous when the heat transfer fluid can hold a temperature once reached for as long as possible without loss, in particular when multiple geothermal probes are connected in series. Especially large-volume geothermal probes are suitable for this. In the case of a larger meter volume, a larger quantity of heat is necessary in order to achieve a higher temperature. However, once it has reached a temperature it offers the advantage of longer temperature stability for the same reason.
Large-volume geothermal probes are characterized in that an inner diameter of the inlet pipe limited by an inlet pipe inner surface and an outer diameter of the outlet pipe limited by the outlet pipe outer surface are selected so that in the annular space a meter volume >10 l is obtained and the heat transfer fluid without further ado is a substantially laminar flow within the annular space. Whether a flow is laminar depends on the geometry of the flow path, the viscosity of the heat transfer fluid and on the flow velocity. The so-called Reynolds number which is a dimension regarding the point at which turbulences occur in a flow is obtained from this. It is generally true that the higher the flow velocity the sooner is the critical Reynolds number exceeded. A low flow velocity ensures an (approximately) laminar flow. In addition, the heat transfer fluid through the low flow velocity has more time of absorbing the heat on the inlet pipe inner surface. A laminar flow however can be considered a disadvantage since within an (approximately) laminar flow layers of uneven temperature are formed. The layers of uneven temperatures run vertically and thus parallel to the inlet pipe inner surface, wherein near the inlet pipe inner surface warmer layers are present, which act as insulators of the inner layers.